Sascha Hoch

Solar Hydrogen Generation with Wide‐Band‐Gap Semiconductors: GaP(100) Photoelectrodes and Surface Modification

GaP, with its large band gap of 2.26 eV (indirect) and 2.78 eV (direct), is a very promising candidate for direct photoelectrochemical water splitting. Herein, p-GaP(100) is investigated as a photocathode for hydrogen generation. The samples are characterized after each preparation step regarding how their photoelectrochemical behavior is influenced by surface composition and structure using a combination of electrochemical and surface-science preparation and characterization techniques. The formation of an Ohmic back contact employing an annealed gold layer and the removal of the native oxides using various etchants are studied. It turns out that the latter has a pronounced effect on the surface composition and structure and therefore also on the electronic properties of the interface. The formation of a thin Ga(2)O(3) buffer layer on the p-GaP(100) surface does not lead to a clear improvement in the photoelectrochemical efficiency, neither do Pt nanocatalyst particles deposited on top of the buffer layer. This behavior can be understood by the electronic structure of these layers, which is not well suited for an efficient charge transfer from the absorber to the electrolyte. First experiments show that the efficiency can be considerably improved by employing a thin GaN layer as a buffer layer on top of the p-GaP(100) surface.

Insights into the Mechanism of Photocatalytic Water Reduction by DFT‐Supported In Situ EPR/Raman Spectroscopy

Considering the foreseeable shortage of fossil resources and global warming, the development of sustainable-energy technologies is of vital interest. An attractive option for the production of more benign energy vectors is the generation of hydrogen by photocatalytic water reduction. This concept facilitates the transformation of sunlight as the ultimate energy source into transportable energy carriers such as hydrogen. Hence, significant efforts are currently being undertaken to increase the activity and stability of suitable water-splitting catalysts. 3] The overall process can be divided into the two half reactions: water oxidation and water reduction. Studying these half reactions in detail, in particular the formation, operation, and decomposition of the catalyst, provides essential information for the development of new more efficient and environmentally benign catalysts. Recently, the Beller group disclosed an efficient water-reduction catalyst system consisting of [Ir(ppy)2(bpy)]PF6 (ppy = 2-phenylpyridine, bpy = 2,2’-bipyridine) as photosenzitizer (IrPS), [Fe3(CO)12] as water-reduction catalyst (WRC), and triethylamine (TEA) as sacrificial reductant (SR; Scheme 1). It is supposed that the catalytic cycle starts by photoexcitation of IrPS and charge separation, and subsequent reduction of its excited state by TEA (SR, cycle I). From the reduced state IrPS an electron is transferred to the WRC, which subsequently reduces aqueous protons to H2 (cycle II). To date, the only intermediate that has been experimentally identified by in situ IR spectroscopy in the water-reduction cascade (Scheme 1) is the anion [HFe3(CO)11] , which is considered to be the catalytically active species. However, the preceding steps leading to its formation as well as pathways responsible for the observed deactivation with time are still not known. Thus, more comprehensive in situ studies using additional methods are highly desired. It is probable that the one-electron-transfer processes in the catalytic cycles I and II (Scheme 1) lead to paramagnetic radical intermediates. Such species are accessible by EPR spectroscopy, while the diamagnetic [HFe3(CO)11] anion is EPR-silent but can be observed by vibrational in situ spectroscopic methods. To gain a more detailed insight into catalytic cycles I and II and to identify possible deactivation processes, we have monitored the reaction simultaneously by in situ EPR/Raman spectroscopy. To the best of our knowledge, photocatalytic water-splitting reactions have never been studied by these coupled techniques. The interpretation of our experimental data is supported by DFT calculations and additional in situ IR studies. First, catalytic cycle I was investigated. As expected, the IrPS complex (low-spin d, diamagnetic) showed no EPR signal in a solution containing THF/TEA/H2O (8:2:1) in the absence of [Fe3(CO)12] without light irradiation. However, if this solution is irradiated at 300 K, an intense isotropic signal at g = 1.9840 is observed (Figure 1). This signal corresponds to the reduced form of the iridium photosensitizer (IrPS ), which is formed by reductive quenching of the excited state (IrPS*) by TEA. A similar signal was formed neither in pure THF nor in THF/H2O, suggesting that 1) TEA is needed as a reducing agent and 2) excitation by light is essential to initiate the electron transfer. However, it must also be mentioned that the signal rapidly declines with time, probably because of ligand dissociation from IrPS (for additional information see Figure SI1 in the Supporting Information). In a reaction mixture containing all the necessary components of the waterreduction system (THF, H2O, TEA, IrPS, and Fe-WRC), no Scheme 1. General principle of H2 formation through the photocatalytic water-reduction cascade.

Advanced Evaluation of the Long-Term Stability of Oxygen Evolution Electrocatalysts

Evaluation of the long-term stability of electrocatalysts is typically performed using galvanostatic polarization at a predefined current density. A stable or insignificant increase in the applied potential is usually interpreted as high long-term stability of the tested catalyst. However, effects such as (i) electrochemical degradation of a catalyst due to its oxidation, (ii) blocking of the catalyst surface by evolved gas bubbles, and (iii) detachment of the catalyst from the electrode surface may lead to a decrease of the catalysts active surface area being exposed to the electrolyte. In order to separate these effects and to evaluate the true electrochemical degradation of electrocatalysts, an advanced evaluation protocol based on subsequently performed electrochemical impedance, double layer capacitance, cyclic voltammetry, and galvanostatic polarization measurements was developed and used to evaluate the degradation of IrO2 particles drop-coated on glassy carbon rotating disk electrode using Nafion as a binder. A flow-through electrochemical cell was developed enabling circulation of the electrolyte leading to an efficient removal of evolved oxygen bubbles even at high current densities of up to 250 mA/cm(2). The degradation rate of IrO2 was evaluated over 225 test cycles (0.733 ± 0.022 mV/h) with a total duration of galvanostatic polarization measurements of over 55 h.

Silicon based tandem cells: novel photocathodes for hydrogen production

A photovoltaic tandem cell made of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) was investigated as a photocathode for hydrogen evolution in a photoelectrochemical device. The electronic and electrochemical properties of the samples were characterized using X-ray photoemission spectroscopy (XPS) and cyclic voltammetry (CV), whereas the morphology of the surface in contact with the electrolyte was investigated by scanning electron microscopy (SEM). The electric efficiency of the tandem cell was determined to be 5.2% in a photoelectrochemical (PEC) setup in acidic solution which is only about half of the photovoltaic efficiency of the tandem cell. A significant improvement in efficiency was achieved with platinum as a catalyst which was deposited by physical vapour deposition (PVD) under ultra-high vacuum (UHV) conditions.